I just installed two of the Pac-audio controllers that Matt lists above (one for the tower speakers and one for the interior speakers), this allows me to completely control the output of all the speakers. I would definitely recommend them. I will post some pics of the install a little later tonight if you would like.

What Brandon said. I just added 2 PAC LC1's in the mounting plate right under my remote. One is for the tower speaks and the other is for the sub. Super clean look and works perfect. No pic yet but I'll get some.